The present invention relates to power tools and in particular to a method and apparatus for controlling the rotational speed of an electronically-controlled drill, hammer-drill or rotary hammer. The control circuit of the present invention provides a controlled start-up routine whereby the rotational speed of the motor is first raised from zero to a predeteremined start-drill speed and thereafter from the start-drill speed to the working speed. The invention is also applicable to a drill, hammer-drill or rotary hammer equipped with an on-off switch and with an electronic control circuit connected to the motor for adjusting the start-drill speed and the working speed.
German Pat. No. 20 55 859 discloses an arrangement for a drill or hammer-drill wherein the rotational speed is increased from zero to the adjusted start-drill speed by actuating an on-off switch having two on-positions from the off position. In the first on-position, the operator can operate the tool in the start-drill mode. In order to bring the tool from the start-drill speed to the working speed, the operator must bring the on-off switch manually into the second on-position wherein a potentiometer tap is rendered operative. The position of the potentiometer tap determines the working speed so that the speed is increased when the switch is switched into the second on-position.
Thus, according to the known method and apparatus, the transition from the start-drill speed to the working speed must be achieved by a manual actuation. Accordingly, it is an object of the invention to simplify the transition from the start-drill speed to the working speed.
To achieve this object of the invention, a method of the invention provides that the rotational speed is automatically held at the start-drill speed for a predetermined time period whereby the speed is increased to the start-drill speed at a constant acceleration and from the start-drill speed to the working speed with the same acceleration.
According to the method of the invention, the ascent of the rotational speed is automatically interrupted for a predetermined time interval when the start-drill speed is reached so that the operator has this time interval at his disposal for starting a bore in the work. Thereafter, the speed increases automatically so that the operator does not have to actuate a switch or take any other action.
In addition, a drill, hammer-drill or rotary hammer equipped with an electronic control circuit according to the present invention incorporates a control unit for interrupting the ascent of the rotational speed for a predetermined time interval when it reaches the start-drill speed. This time interval is preferably adjustable so that the operator can select the length of time during which the start-drill speed is to be available to him in dependence upon the particular application required.
In order that the portable tool can be utilized without interrupting the ascent of the rotational speed, the tool can be equipped with an on-off switch having two on-positions. In the first on position, the switch can activate the control unit and, in the second on-position the interruption of the ascent in rotational speed can be made ineffective so that by a movement of the on-off switch from the off position directly to the second on position, the rotational speed will continue to increase uninterrupted from zero to the working speed.
A further object of the present invention is to provide a drill, hammer-drill, or rotary hammer incorporating a speed control apparatus as described herein which is relatively inexpensive and easily installed during assembly of the tool so as to minimize the incremental cost of manufacturing the tool.
To achieve this object of the invention, the solid-state electronic control circuitry according to the present invention is contained within a housing located above the motor which mates with the left-half and right-half outer housing members and serves as the top surface of the tool housing. The forward end of the control circuit housing includes an integral depending portion for supporting the speed sensing coil in operative relationship with the magnet coupled to the output shaft of the motor between the armature and the cooling fan. In this manner, the installation of the control circuit housing automatically properly positions the speed sensing coil relative to the magnet, thereby simplifying assembly of the tool.